1. Field of the Invention
The present invention relates to a method of storing volatile substances. More particularly this invention, when substances that are gaseous at room temperature are stored in a liquid state or a solid state, is concerned with a method of storing volatile substances where a minimum loss of the substances by vaporization can be achieved.
The present invention also relates to a container suited for storing liquid helium.
The present invention further relates to a method of controlling superfluid surface flow of a superfluid liquid upon utilizing the superfluid liquid.
2. Related Background Art
Hitherto, volatile substances have been stored by putting them in closed top containers, stored by putting them in high pressure bombs, or stored by cooling them.
In general, many methods of storing volatile substances that are solid or liquid at room temperature have been known.
However, there is a limit in methods of storing in a liquid or solid state the substances that are gaseous at room temperature.
For example, substances such as liquefied natural gas, liquid air, liquid nitrogen, liquid oxygen, liquid hydrogen and liquid helium have a high saturated vapor pressure at room temperature, so that they are usually stored under pressure or cooling.
Accordingly, to keep these substances in a stable state for a long period of time, great efforts have been made on how the temperature is kept low and also how heat is prevented from coming in.
For storing these substances, commonly used are Dewar vessels or other thermal insulating containers.
Of these substances, greater efforts are made for storing liquid helium.
This is because liquid helium has properties different from general liquefied gases.
More specifically, liquid helium has a very low boiling point under 1 atomospheric pressure, and helium-4(.sup.4 He) liquefies at about 4.2.degree. K. and helium-3(.sup.3 He) liquefies at about 3.2.degree. K. For this reason, it is necessary for storing the liquid helium to use a container made of a thermal insulating material such as metal (e.g., stainless steel) or glass having a low thermal conductivity, and used are containers whose walls are further doubled and inside of the double walls are stored vacuum, which are the so-called Dewar vessels.
Usually, in instances where the Dewar vessels are used, methods are taken such that the inside thereof is made to have a double structure, liquid helium is poured in an inner Dewar vessel, and liquid nitrogen or the like is filled between the inner Dewar vessel and an outer Dewar vessel, thus suppressing the inflow of heat by thermal radiation and conduction.
In the conventional methods as described above, however, it has been unavoidable to suffer a loss due to unnecessary vaporization when the substances that are gaseous under a normal pressure are liquefied or solidified for storing, particularly when they are stored for a long period of time.
Particularly in the case of liquid helium, and taking into account the expense involved, loss due to vaporization is a problem that should be overcome.
Also, when liquid helium is being cooled, it transforms into a peculiar state that it is in a superfluid state below a certain temperature. Helium-4(.sup.4 He) transforms into superfluid state at 2.17.degree. K. or less, and helium-3(.sup.3 He) transforms thereinto at a far lower temperature. Once liquid helium turns to a superfluid state, it loses its viscosity and endlessly extends in the form of a superfluid surface flow along the surface of the container or other structures coming into contact with the liquid helium. This surface flow can not be stopped if, for example, a mouth of the container is simply stoppered or so, and overflows from even a small gap between the stopper and the container without any resistance at all. To stop such a surface flow, the container is required to be hermetically closed in an atomic scale, and the stopping of the surface flow is very difficult if a reusable mechanical sealing means is applied, resulting in an unavoidable loss of liquid helium.
It has been hitherto considered impossible to prevent or control such a superfluid surface flow of liquid helium. For this reason, it only has been practiced within a very limited scope to carry out cooling by utilizing a high thermal conductivity in superfluidity, keeping the liquid helium stable as being cooled to about 1.degree. K., or efficiently transporting liquid helium by utilizing superfluidity.
It has been deemed impossible in theory to limit the surface flow to a particular region in a stable state on the surface of an article coming into contact with superfluid helium, or to control the magnitude or velocity of the surface flow, and thus there has been no choice but to allow it to flow.